What Is A Search Engine Optimisation? A search engine is a computer program that searches documents for specified keywords and returns a list of the documents where the keywords matched the search criteria. Although search engine is really a general class of program, the term is repeatedly used to specifically describe systems like Google, Alta Vista and Excite, that enable users to search for information on the World Wide Web. In general, a search engine works by sending out a spider to fetch and return as many documents as possible. Another program, called an indexer, then reads these documents and creates an index based on the words contained in each document. Each search provider uses a unique proprietary algorithm to create its indices such that, in an ideal world, only meaningful results are returned for each query presented to the search engine.